Magnetic resonance imaging (MRI) and nuclear magnetic resonance imaging (NMRI) are medical imaging techniques used to visualize certain structural aspects and functionality of human and animal subjects. The imaging typically uses a magnetic field to align certain atoms in the body in the direction of the magnetic field. Changes to the alignment cause a rotating magnetic field that can be detected. Contrast agents can be used to enhance imaging by, e.g., providing additional types of atoms for imaging. Variants of magnetic imaging are known in the art.
In a standard imaging acquisition, there is a prescan process for determining the optimal parameters to be used for the scan process. Prescan has multiple steps, such as the determination of the exact scanner carrier frequency, receiver gains, and signal amplification (or transmit gain), all of which are used to insure maximum detected signal. The process of prescan is typically performed at the same frequency as the frequency of the imaging experiment.
The use of hyperpolarized molecules in conjunction with magnetic resonance imaging has advanced magnetic resonance imaging into the realm of metabolic imaging. Typically, in an exam in which hyperpolarized compounds labeled with 13C atoms are injected in a subject, two coils are used, one tuned at the proton frequency and one at the carbon frequency. In some examples, a single, dual tuned coil is used.
One difficulty when performing imaging in the presence of 13C, hyperpolarized compounds is the calibration of the system such that optimal imaging is performed. Prior to the injection of the hyperpolarized compounds, there is typically insufficient naturally occurring 13C signal that can be used for flip angle calibration. The labeled compound is typically injected into the subject, and images of anatomical distribution of the injected hyperpolarized compound and its downstream products are obtained. The signal from the injected compound is time varying; it changes in time due to relaxation, flow, perfusion and metabolism. For minimal signal loss to occur, it is important that imaging takes place soon after the injection occurs without lengthy procedures for calibration.
Unless a lipid-rich area exists in the region to be imaged with enough natural abundance 13C, flip angle calibration is usually not performed in vivo. In some cases, the calibration is initially performed in a phantom designed to mimic the in vivo subjects. In these phantom calibrations, the value for the transmit gain is obtained and used for all the in vivo exams, ignoring the different loading provided by different subjects. Alternatively, a phantom containing carbon enriched material can be added in the imaging region, and flip angle (FA) calibration is performed using the signal from this phantom. However, since this phantom has to be inserted in the coil in the presence of the subject, it has to be small, leading to some inaccuracies in FA calibration due to limited signal availability from the small phantom sample. Moreover, such a phantom has to be inherently positioned near the coil, where the radio-frequency magnetic field (B1) tends to be inhomogeneous, and potentially significantly different than the B1 in the center of the coil, where the subject sits. Consequently, conventional approaches for FA calibration are not particularly accurate and can lead to undesired signal loss. Improved techniques for calibration would enhance the hyperpolarized imaging technology and make the systems more commercially attractive.